SIMS (Secondary Ion Mass Spectrometry) has been used as means capable of quickly analyzing the concentrations of impurities at high sensitivities for evaluating many electronic materials including send conductor materials. As shown in FIG. 1, in SIMS, a primary ion is irradiated on a sample placed in a vacuum chamber to eject atoms or atomic clusters from the surface of the sample (hereinafter, referred to as sputtering). A secondary ion thus generated is measured and the concentration of the impurity in the sample is then calculated.
Next, a raster variation method will be described. The raster variation method is a technique frequently employed in the process to carry out a SIMS analysis on an atmospheric component element (such as H, C, N, or O). When the SIMS analysis is carried on such an atmospheric component element, a background is generated due to any adsorption component on the surface of the sample or the inner wall of the chamber, residual gas in vacuo, or the like. The background may contribute to the detected signal of the secondary ion of the impurity element which is the target of the measurement, thereby degrading the lower detection limit of the measured concentration of the impurity element, or destabilising the detected signal. The raster variation method carries out the measurement of a secondary ion with respect to each of the primary component substance and the impurity element two times while changing the irradiation density of the primary ion to allow the contribution of the background to be separated. If the background can be calculated as described above, the contribution thereof can be canceled from the detection signal so that the concentration of an impurity that is lower than that of the background can be calculated.
[Non-Patent Document 1] TORAY Research Center, “Microanalysis of Atmospheric Component Element in Semiconductor Material” [online]; [retrieved on Jun. 3, 2004], Internet <URL: http://www.toray-research.co.jp/sims/pdf/taikiseibun.pdf>.